This invention relates to integrated circuits, and more particularly to methods and automated systems for efficiently testing integrated circuits.
Molded IC devices are often assembled (packaged) on matrix-type lead frame structures in which the IC devices are arranged in multiple rows and columns, and then tested while connected to the matrix-type lead frame (i.e., before being singulated (separated) into individual IC devices). As utilized herein, the term xe2x80x9cassembly stripxe2x80x9d is used to describe the integral structure formed by such a matrix-type lead frame structure with IC devices packaged thereon. Assembly strips facilitate low-cost automated production by allowing several IC devices to be tested simultaneously (i.e., in parallel), thereby reducing manufacturing time and costs.
FIGS. 1(A) through 1(C) are perspective views showing a conventional process of assembling IC devices using a lead frame 100, which is simplified for descriptive purposes. Referring to FIG. 1(A), lead frame 100 is etched or stamped from a thin sheet metal strip, and includes side rails 110, cross rails 120, and multiple die attach regions 130. Each die attach region 130 includes a die attach platform 132 connected to side rails 110 by tie bars 135, and patterns of narrow leads 140 that radiate inward from side rails 110 and cross rails 120 toward die attach platform 132. Note that leads 140 do not contact die attach platform 132. During a first stage of the bonding process that is shown in FIG. 1(A), an IC die 150 is mounted onto each die attach platform 132 using, for example, an epoxy resin. A pattern of die bond pads. 152 are provided on an upper surface of IC die 150 that are electrically connected to the integrated circuit formed thereon. As shown in FIG. 1(B), after IC die 150 is secured to die attach platform 132, each die attach platform 152 is :electrically connected to a corresponding lead 140 by a fine-diameter gold bond wire 160 using well-established wire; bond techniques. Subsequently, as indicated in FIG. 1(C), die attach platform 132, the inner ends of leads 140, die 150, and bond wires 160 are covered with a thermoset plastic casing 170 during a transfer molding operation. Note that a portion of each lead 140 is exposed along the sides of casing 170. The integral structure including lead frame 100 and the fully packaged IC device is referred to below as assembly strip 105.
FIGS. 2(A) through 2(C) show a conventional process for functional testing, lead formation, and singulation (i.e., separation of individual IC devices 200 from assembly strip 105), which is performed after the assembly process shown in FIGS. 1(A) through 1(C). First, as shown in FIG. 2(A), the conventional process includes cutting leads 140 such that they are separated from side rails 110 and cross rails 120 of lead frame 100. Note that IC devices 200 remain connected to assembly strip 105 by tie bars 135, and that leads 140 remain flattened (i.e., in plane with side rails 110 and cross rails 120 of lead frame 100). As shown in FIG. 2(B), functional testing is then performed during which test signals are transmitted from a tester 210 to IC devices 200 via probes 215, which are pressed against leads 140 by a suitable mechanism. Note that functional testing is performed while leads 140 are flat (i.e., in the plane defined by lead frame 100). Finally, as indicated in FIG. 2(C), lead forming and singulation is performed to produce individual IC devices 200 having fully formed leads 140. After singulation, lead frame 100 is discarded.
A problem with the conventional testing and singulation process shown in FIGS. 2(A) through 2(C) is that three separate systems are required to perform each of lead cutting (FIG. 2(A)), functional testing (FIG. 2(B)), and singulation (FIG. 2(C)), thereby increasing the total production cost per IC device 200. Further, transferring assembly strips 105 between these separate systems inevitably leads to accidents that damage IC devices 200, further increasing production costs.
What is needed is an efficient and cost effective system and method for performing functional testing, lead formation, and singulation of IC devices that avoids the cost and handling issues associated with the conventional methods described above.
The present invention is directed to a method for processing and testing ICs mounted on an assembly strip in which both functional and visual lead inspection are performed after cutting and forming the leads, and prior to singulation (i.e., prior to separation of the individual ICs from the assembly strip). Accordingly, the present invention facilitates functional testing, lead formation, and singulation using a single, relatively inexpensive system, thereby reducing overall production costs when compared to conventional methods for performing these procedures. Further, because the assembly strips remain attached to a single system throughout functional testing, lead formation, and singulation, the present invention also minimizes handling by eliminating transfer between independent systems, thereby reducing the costs associated with damage caused during such transfers.
Each assembly strip processed in accordance with the present invention includes multiple rows and columns (e.g., 5xc3x9712) of ICs that are mounted on a matrix-type lead frame. In one embodiment, the lead frame includes IC mounting regions made up of a die attach platform that is connected at opposite ends to the lead frame, and parallel leads extending from opposing sides of the die attach platform to lead tie bars of the lead frame. An IC is mounted on each die attach platform and connected (e.g., using wire bonding techniques) to the leads located adjacent to the die attach platform. Subsequently, packaging material (e.g., thermoset plastic) is formed over the IC, bonding wires and die attach platform.
In accordance with a disclosed embodiment of the present invention, a method for processing ICs mounted on assembly strips utilizes a lead length cut/form apparatus, a functional test apparatus, and a singulation apparatus. After the IC dies are mounted on the assembly strip, they are loaded into magazines and systematically moved by an onloader to a conveyor, which moves the assembly strips to the lead length cut/form apparatus. The lead length cut/form apparatus cuts the leads connected to the package of each IC, preforms (i.e., bends) the leads, and forms the leads into a desired final form without separating the ICs from the assembly strip. The assembly strips are then passed to the functional test module in which probes (e.g., pogo pins) are pressed against the fully formed leads and functional tests are transmitted to the ICs from a tester. Visual inspection of the leads is then performed to identify defective leads, e.g., damaged leads, bent leads, or missing leads. After functional and visual testing, the assembly strips are passed to a singulation apparatus that separates the ICs from the assembly strip frame, and to an offloader that loads the separated ICs into storage tubes. An optional second visual inspection may be performed after singulation and prior to loading to detect package defects that may have occurred during the testing and singulation operations, or during preceding processes.
In accordance with an aspect of the present invention, the lead cut/form process and singulation process are performed using a single drive apparatus, thereby reducing costs by eliminating the need for separate drive mechanisms for these two operations. Note that the functional testing process, which is performed between the lead length cut/form and singulation processes, is provided with a separate ball-screw drive that facilitates testing of the ICs. A conveyor is utilized to automatically pass each assembly strip from the lead length cut/form process to the functional test process, and from the functional test process to the singulation process, thereby minimizing IC damage caused by transporting the assembly strips between separate systems.
In accordance with another aspect of the present invention, functional testing is performed using a stationary anvil and a probe assembly that is moved toward and away from the anvil by the ball-screw drive. The anvil includes a trough and a pair of rails that hold the IC devices during testing. The probe assembly includes probes (e.g., pogo pins) that are pressed against the leads of the IC devices, which are supported by the rails to prevent damage. The probes are arranged to include a first set positioned to contact a portion of the leads located on top of the rails when the probe assembly is initially moved toward the anvil, and a second set positioned to contact the feet of the leads when the probe assembly is moved further toward the anvil. Accordingly, the functional testing module facilitates functional testing while preventing damage to the leads, thereby reducing the number of systems needed to perform the testing and singulation process.